The present invention relates to industrial process control and monitoring systems in the measurement of process variables and such systems. More specifically, the present invention relates to measurement of process variables to process fluid related to density of the fluid.
Industrial processes are used to monitor and/or control operation of industrial processes. For example, process variable transmitters are used to measure process variables of the industrial process. Examples of process variables include temperature, pressure, level and flow. Level of fluid in a tank of flow of a process fluid through a pipe, such as mass flow, are typically not measured directly but are calculated from a number of process variables.
For example, in industrial processes, transmitters which measure flow rate (Q) are placed at remote locations in the field of a process control system. These transmitters transmit flow rate information to a control room. The flow rate information is used to control operation of the process. As used herein, process fluid refers to both liquid and gaseous fluids.
One common means of measuring flow rate in the process control industry is to measure the pressure drop across a fixed restriction in the pipe, often referred to as a differential producer or primary element. The general equation for calculating flow rate through a differential producer can be written as:Q=NCdEY1d2√{square root over (ρh)}  Equation 1Where:    Q=Mass flow rate (mass/unit time)    N=Units conversion factor (units vary)    Cd=Discharge coefficient (dimensionless)    E=Velocity of approach factor (dimensionless)    Y1=Gas expansion factor (dimensionless)    d=Bore of differential producer (length)    ρ=Fluid density (mass/unit volume)    h=Differential pressure (force/unit area)
Of the terms in this expression, only the units conversion factor, which is a constant, is simple to calculate. The other terms are expressed by equations that range from relatively simple to very complex. Some of the expressions contain many terms and require the raising of numbers to non-integer powers. This is a computationally intensive operation.
There are a number of types of meters which can be used to measure flow. Head meters are the most common type of meter used to measure fluid flow rates. They measure fluid flow indirectly by creating and measuring a differential pressure by means of an obstruction to the fluid flow. Using well-established conversion coefficients which depend on the type of head meter used and the diameter of the pipe, a measurement of the differential pressure may be translated into a mass or volume rate.
One technique for measuring a differential pressure for determining flow is through an averaging pitot tube type primary element. In general, an averaging pitot tube type primary element for indicating flow consists of two hollow tubes that sense the pressure at different places within the pipe. These tubes can be mounted separately in the pipe or installed together in one casing as a single device. An example of an averaging pitot tube is shown in U.S. Pat. No. 4,154,100, entitled METHOD AND APPARATUS FOR STABILIZING THE FLOW COEFFICIENT FOR PITOT-TYPE FLOWMETERS WITH A DOWNSTREAM-FACING PORT. This design includes a forward facing tube which measures total pressure (PTOT). A second tube measures a down stream pressure. The differential pressure between the two tubes is proportional to the square of the flow as given in Equation 2.Q=NKD2Y1√{square root over (ρh)}  Equation 2where:    N=Units conversion factor    K=flow coefficient of the averaging pitot (dimensionless)    D=Pipe diameter (inches)    Y1=Gas expansion factor (dimensionless)    ρ=Gas density (lbm/ft3)    h=Differential pressure (inches H2)
Accurate calculation of flow based upon pressure measurement requires accurate measurement of static pressure in order to determine density (ρ) and the gas expansion factor (Y1) for use in Equation 1. However, the additional sensor to sense static pressure (PSTAT) in the prior art is cumbersome, inconvenient, expensive and provides an additional source of errors.